The density of the semiconductor elements mounted on a wiring substrate has increased. Now, there is a demand for wiring substrates that are thinner and wiring patterns that are increased in density. A coreless wiring substrate has been proposed to satisfy such demands. A coreless substrate is less a core substrate that has high rigidity and is thicker than an interlayer insulation layer. Such a wiring substrate easily warps and is difficult to handle during manufacturing processes and assembling processes. Japanese Laid-Open Patent Publication No. 2016-048768 describes the manufacturing and assembling of a wiring substrate in a state in which the wiring substrate is adhered to a rigid tentative substrate.